Tomato (Solanum lycopersicum, also called Lycopersicon esculentum) is a plant of the Solanaceae or nightshade family. It is a short-lived perennial plant, grown as an annual plant, and a close relative of the potato. The fruit (i.e. the tomato) is an edible, brightly coloured (usually red, from the pigment lycopene) berry, 1-2 cm diameter in wild plants, commonly much larger in cultivated forms. The plant is now grown worldwide for its edible fruits, which are major fresh market vegetables worldwide.
The growth habit of tomato plants is commonly classified as determinate or indeterminate. This classification depends on the capacity of the shoot system for continued sympodial growth. Indeterminate cultivars (where the apical meristem grows indefinitely and flowers arise from the axillary meristem) produce branched systems that grow indefinitely, whereas determinate cultivars (where apical meristem is converted into terminal flower) produce branching systems with progressively fewer nodes until the shoot terminates with two inflorescences and develops the form of a bush. This change in plant architecture is due to a mutation in the SELF-PRUNING (SP) gene (Pnueli et al., Development 125: 1979-1989, 1998) and has been an important development for this crop, because determinate types can be harvested mechanically and are therefore mainly used for the processing industry, whereas indeterminate types are generally grown in greenhouses and are used for the fresh market.
Fridman et al. (Mol. Genet. Genomics 266: 821-826, 2002) previously have shown that introgression of a S. Pennellii allele of a QTL named PW9-2-5 in a sp/sp S. Esculentum background results in a semi-determinate growth with 2 leaves between the trusses (designated as spi=2). They suggested that the SP9D gene from S. Penellii, SP9Dpen, is the candidate gene for the change of plant architecture and also the so-called Solid Solid Content (SSC) or the refractive index (which is indirectly related to the taste). The SP9D gene belongs to the CETS gene family (CENTRORADIALIS, Terminal Flower) and it is believed that this gene family plays a key role in determining plant architecture (Carmel-Goren, Plant Mol. Biol. 52; 1215-1222, 2003).
S. lycopersicum has six CETS gene family members, named SP, distributed over five different chromosomes: SP2I, SP3D, SP5G, SP6A, and SP9D, wherein the names are given according to the bin position (Pan et al., Genetics 155: 309-322, 2000). The sixth member, SP, is located on chromosome 6, bin E, and is known to be the gene that alters the tomato in a determinate (sp/sp)/indeterminate (SP/−) phenotype. Phylogenetic relationships grouped SP3D, SP5G, and SP6A with the Arabidopsis FT gene. SP9D and SP are grouped with Arabidopsis TFL-1, and SP2I is in the same branch with the Mother of Flowering Time (MFT) of Arabidopsis (Carmel-Goren et al., supra). Despite the phylogenetic relationships between the genes, the expression profiles differ. Thus, SP5G expression has been found predominantly in cotelydons, whereas SP3D is expressed mainly in floral organs with low expression in vegetative organ leaves. For SP6A no expression
has been found sofar. SP9D is mainly expressed in the shoot apex and has a high expression in roots, whereas SP2I is expressed in all organs tested. In spite of these expression profiles little is known about their function with SP as an exception.
During the last decades, indeterminate tomato breeding was mainly focussed on yield, disease resistance, and fruit quality aspects such as uniform ripening and taste. Yield improvements have been achieved due to new production methods, improved pest management and varieties that are better suited for new production methods, but the gains in yield become smaller. New varieties with 5 or 15 fruits more per plant gave a yield increase of 2-4%.
Development of varieties with higher yield was hampered by the lack of knowledge regarding the aspects that determine tomato yield. Xiao (2004; ISHS Acta Horticulturae 654 (International workshop on models for plant growth and control of product quality in horticultural production) and Heuvelink (2005; ISHS Acta Horticulturae 691 (International Conference on sustainable greenhouse systems—greensys2004; 43-50) simulated that a tomato variety with two leaves between trusses instead of the conventional three leaves would shift assimilation towards the fruits, resulting in higher yields when the Leaf Area Index (LAI) is maintained. They validated the simulated data by removing every second leaf and keeping the LAI above 3. As simulated, the yield increased by roughly 10%.
Cultivated varieties with two leaves between the trusses are not known, however, there are wild tomato relatives with two leaves between the trusses, i.e. having a sympodial index=2 (spi=2), such as Solanum neorickii, Solanum chmielewskii, Solanum chilense, Solanum peruvianum, and Solanum pennellii. The property of a sympodial index of 2 is recessive to the sympodial index of 3 in cultivated tomatoes of F1 hybrids of the inter-species cross with S. penellii (Pnueli et al., 1998, supra). The genetic basis of spi=2, however, sofar is not known.
As the world population continues to grow, the demand for fresh vegetables, such as tomatoes, is ever increasing worldwide. Thus, a continuing need exists for means and methods for improving yield of vegetable plants, such as tomato.